I don't have experience with the iPhone 6s +dbrand + Apple's case, but from experience with skins on other devices, I've had zero issues with fitments. The skins themselves are really thin and don't add much (if any) extra thickness to the device, allowing for most (if not all) cases to fit just fine.

MacRumors attracts a broad audience
of both consumers and professionals interested in
the latest technologies and products. We also boast an active community focused on
purchasing decisions and technical aspects of the iPhone, iPod, iPad, and Mac platforms.